Battery separator material

ABSTRACT

A novel, improved battery separator material particularly adaptable for use in maintenance free batteries. The battery separator material includes a diatomaceous earth filler, an acrylate copolymer binder and a combination of fibers comprising polyolefin, polyester and glass fibers.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 445,857, filed Dec. 12, 1982 now abandoned which in turn is acontinuation-in-part of U.S. patent application Ser. No. 345,173, filedFeb. 2, 1982 and now abandoned.

BACKGROUND OF THE INVENTION

1. The Field Of The Invention

This invention relates to battery separator materials. More precisely,this invention relates to relatively inexpensive, non-cellulosic, lowohmic resistance, maintenance free battery separators.

2. Description Of The Prior Art

Compact portable electrolytic cells or batteries including spaced apartmetal plates connected in series for storage of electrical energy arewell known to the art. An important factor involved in the developmentof such batteries is the use of thin, porous separators placed betweenalternating positive and negative plates. The primary function assignedthe separator is to effectively prevent metallic conduction between theplates to avoid short circuiting. At the same time, the separator mustpermit free passage of electrolyte solution for effective ion exchangebetween the plates.

Commercial separators have been manufactured from wood, microporousrubber, fiberous glass mats, microporous plastics, cellulosic andfiberous materials impregnated with acid insoluble resins and syntheticand glass woven fiberous and porous vitrious materials. Additionaldetails relating to battery separators can be found in U.S. Pat. Nos.2,793,398; 3,890,184; 4,024,323; 4,055,711; 4,113,927; 3,753,784;3,694,265; 3,773,590; 3,351,495; 3,340,100; 3,055,966; 3,205,098 and2,978,529.

The growing evolution of the so-called "maintenance free" battery haspresented special considerations and problems in the development ofbattery separators. The maintenance free battery is a sealed unit whichdoes not require addition of water during the life of the battery. Formany years, the separator industry has preferentially producedcellulosic separators impregnated or treated with phenol aldehydes.Phenol aldehyde impregnated cellulosic separators are described indetail in U.S. Pat. Nos. 3,272,657 and 3,247,025. These cellulosicseparators presented an especially desirable combination of commerciallyattractive features including excellent electrical performancecharacteristics, low cost and good physical manufacturing properties.However, the outgassing properties in the cellulosic/phenolic typeseparator present problems in the maintenance free battery and thisproperty has effectively disqualified phenol aldehyde impregnatedcellulosic separators for use in permanently sealed units. Additionally,there has been a recent increase in environmental concern in theindustry with the use of phenolic aldehydes.

Alternative separators have been developed by the industry for themaintenance free battery. However, these alternative separators have notpresented the attractive combination of commercial features which led tothe wide-spread acceptance and use of the earlier phenol aldehydeimpregnated cellulosic separators. Instead, the alternative separatorsare generally associated with one or more commercially unattractivefeatures such as high cost, and/or inferior electrical characteristicsand/or poor physical properties and/or the use of materials offabrication which are leachable or oxidizable thereby providing highchemical oxidation demand (C.O.D.).

In the development of alternative separators, particular emphasis hasbeen placed on non-cellulosic separators providing good oxidationresistance and low ohmic resistance. The resistance to the passage ofthe electric current through the separator is an important factor, andis generally associated with the total porosity of the separator. As anapproximation, the porosity may be assumed to be inversely proportionalto the ohmic resistance. However, the manner in which porosity isachieved is also of special importance. The smaller the pore size, themore the tortuosity of the path of the electrolyte through theseparator, thereby reducing the opportunity for formation of metallicdeposits through the thickness of the separator which can cause a directshort. Porosity is achieved in most rubber and plastic typeseparators--whether formed by coagulation or extrusion techniques--byincluding small particles in the matrix which can be leached out byappropriate solutes after formation of the separator. This processprovides a suitable degree of porosity but is intrinsically both costlyand time consuming. In fiberous sheet separators, porosity is usuallycontrolled by the selection of a variety of fiber diameters to regulatethe size of the pores created at the interstices of the fiber crossoverpoints and also by including relatively small amounts of siliceous typefillers. Separators providing high pore volumes can be achieved by thistechnique. However, the pore size and average pore size is somewhatlarger than desired.

This invention is addressed to the outstanding problem in the art ofproviding a low cost, high performance, non-cellulosic separator formaintenance free batteries and presents an especially effective solutionto the problem.

BRIEF SUMMARY OF THE INVENTION

The invention presents to the art, novel improved battery separatormaterials comprising a combination of selected fiber materials, aselected filler material and selected binder materials. Essentially, theselected fiber material includes a combination of polyolefin, polyesterand glass fibers while the filler is a diatomaceous earth and the binderis an acrylate copolymer. In accordance with the practice of theinvention, the combination of selected materials provides a low cost,non-cellulosic battery separator having an acid wettability of underfive seconds exposure time, a total pore volume of about 70% or higherand providing an average pore size of about 5 to about 30 microns, a lowchemical oxygen demand and an ohmic resistance below about 0.015 ohmsper in². The battery separator materials of this invention areespecially adaptable for use as separators in maintenance freebatteries.

DESCRIPTION OF PREFERRED EMBODIMENTS

The battery separators of this invention comprise a selected combinationof fibrous materials, filler material and binder material(s). Thefiberous materials include a combination of polyolefin fibers, polyesterfibers and glass fibers and the composition of the separator material ispresented in more detail below:

    ______________________________________                                                    Percent By Weight in Separator                                                Range   Preferred Range                                           ______________________________________                                        Polyolefin Fiber                                                                            5-20       8-16                                                 Polyester Fiber                                                                             2-15       5-12                                                 Glass Fiber   0-20      12-18                                                 Filler        40-75     45-60                                                 Binder        7-20      8.5-17                                                ______________________________________                                    

Polyolefin pulp fibers comprise a portion of the fiberous materials ofbattery separators of the invention. The preferred polyolefin pulpfibers are synthesized from the polymerization of ethylene and/orpropylene or mixtures thereof to provide polyethylene, polypropylene orcopolymers of ethyelene and propylene. Preferably, the polyolefin pulpfibers have fiber diameters between about 0.01 to about 20 microns, asoftening point below about 340° F. and are up to about 0.5 inches inlength.

The polyolefin fibers most suitable in the practice of the invention arethose which are characterized as synthetic wood fibers and have asurface treatment which provides improved wettability and dispersabilityin water. Additional properties of the preferred polyolefin fibers are(1) a surface tension of about 70 mN/m; (2) a specific gravity of lessthan one and preferably between about 0.900 to about 0.965 kg/dm³ ; (3)a melting point between about 250° to about 340° F.; (4) a yield stressgreater than 300 daN/cm², (5) a tensile strength at break of greaterthan 200 daN/cm² ; (6) a modulus of elasticity in tension of betweenabout 7000 to about 20,000 daN/cm² and (7) a dielectric constant ofabout 2 to about 4, a dielectric strength of about 2 to about 5×10²kf/cm, and a transverse resistivity of about 10¹⁵ to about 10¹⁸ Ωcm. Thepolyolefin fibers are used in amounts to provide between about 5 toabout 20 percent by weight of the total weight of the finished separatorand most preferably in amounts to provide between about 8 to about 16percent by weight.

Polyester fibers are included in the battery separator materials of theinvention. The polyester fibers are acid resistant and improve theoverall strength of the separator particularly the puncture resistanceand tear strength of the separator material. The most preferredpolyester fibers are polyethylene terephthalate fibers having a denierbetween about 0.5 to about 2.2 and a length of at least about 1/4 inchesand preferably between about 1/4 inches to about 3/4 inches. Thepolyester fibers are used in amounts to provide between about 2 to about15 and most preferably between about 5 to about 12 percent by weight ofthe total weight of the separator. A surfactant may be carried orotherwise associated with the surface of the polyester fiber to improvethe dispersability of the fiber in water.

Acid resistant glass fiber (so-called "Chemical Grade") are preferablyincluded in the separators of this invention to improve the wettabilityand wicking action of the separator and to provide improved stiffness.Suitable glass fibers have diameters ranging between about 2 microns toabout 15 microns and preferably between about 6 microns to about 15microns. The length of the glass fibers may range between about 1/8inches to about 3/4 inches and preferably between about 1/8 inches toabout 5/8 inches. Combinations of glass fibers having different lengthsand/or diameters have been suitably employed in the practice of theinvention. The glass fibers are used in amounts to provide between 0 toabout 20 and preferably between about 12 to about 18 percent by weightof the total weight of the separator. Glass fibers having a length ofabout 1/2 inch and a diameter of about 5-13 microns are particularlypreferred since they provide a desirable degree of stiffness in thefinal separator. The preferred glass fibers of about 1/2 inch in lengthmay be used as the only glass fiber in the separator or they may be usedin combination with other glass fibers having different lengths and/ordiameters. Glass fibers which have been treated with a water dispersablefinish to provide improved dispersability in water are preferred in thepractice of the invention.

The filler used in battery separators of this invention is diatomaceousearth which is also known as diatomite, infusorial earth and kieselguhr.Essentially diatomaceous earth is an amorphous silica formed from thefossilized remains of single celled aquatic plants called diatoms.Diatomaceous earth is characterized by its unique microscopic structure,chemical inertness, high absorptive capacity, internal porosity andgreat bulk per unit weight. The diatomaceous earths are commerciallyavailable in both natural or flux calcined grades. Natural gradediatomaceous earth is somewhat softer than the flux calcined grade andthe flux calcined grade is definitely preferred in the practice of thisinvention. Typical properties of flux calcined grade diatomaceous earthsare listed below:

    ______________________________________                                        Color (dry)               white                                               TAPPI brightness %        89-92                                               Scum (maximum retained on 325 mesh) %                                                                   trace, 3%                                           Absorption.sup.1. (water-average), lbs/100 lb.                                                          160                                                 Moisture content (maximum)                                                                              1.0                                                 pH                        7.0                                                 Specific gravity          2.30                                                ______________________________________                                    

The skeletal or cellular structure of the diatomaeous earth fillerprovides the porosity for the separators of the invention and the costof this porosity generating filler is relatively low. The amount offiller used is important in generating the desired degree of porosityand is sufficient to provide between about 40 to about 75 and preferablybetween about 45 to about 60 percent by weight of the total weight ofthe separator. The preferred amounts provide a highly filled separatorpreferably having a total pore volume greater than about 80 percent withan average pore size between about 5 to about 30 microns, a preferredtypical average pore size is about 22 microns while a preferred typicalpore size distribution involves about 4 percent of the pores having apore size greater than 45 microns, about 69 percent having a sizebetween about 15-45 microns and about 27 percent having a size less than15 microns. Pore size is determined on a mercury intrusion porisimeter.The ohmic resistance of separators of this invention--which asmentioned, is related to pore volume and pore size--is about 0.013 ohmsinch² ±0.003.

The binder included in separators of this invention areself-crosslinking, non-coalescing (at room temperature) acrylatecopolymers in which an acrylate moiety of the following formularepresents about 80 percent or less preferably between about 80 to about30 percent of the backbone moieties: ##STR1## wherein is the integer 1-8or somewhat higher but preferably 1-4.

Particularly suitable polymeric binders are copolymers including methylor ethyl or butyl acrylate moieties with moieties of monomers havinga >C═C< group and which provide acrylate copolymers having glasstransition temperatures between about 30 30° C. to about +60° C. Theglass transition temperatures (T_(g)) are measured by DifferentialThermal Analysis on a Differential Scanning Colorimeter (E. I. duPont DeNemours Model 910) at a heating rate of 10° C./min. Especially suitablecomonomers for binders of this invention are the so-called "hard"monomers such as methyl or ethyl methacrylate and styrene or derivativesof styrene such as alpha methyl styrene. Particularly preferred bindersare methyl methacrylate/acrylate copolymers.

The preferred polymeric binders are further characterized as acrylatecopolymers having a surfactant(s) attached to the copolymer molecule soas to be substantially non-leachable under the conditions of separatorpreparation and operation. Preferably the attached surfactant is ananionic surfactant such as a sulfo-succinate ester present in an amountbetween about 0.25 to about 2.5 percent by weight of the copolymer.Preferred polymeric binders of this invention are further characterizedas having intrinsic viscosities between about 0.8 to about 2.0 at 30° C.Intrinsic viscosities are measured on a freshly prepared copolymer latex(to minimize crosslinking effect) using an amount of the latexsufficient to provide about 0.3 to about 0.4 grams copolymer solid. Theamount is placed in 100 mls dimethyl formamide, allowed to dissolve andflow rates of solution and solvent alone are measured at 30° C. in anOstwald type viscometer.

The especially preferred acrylate copolymers are those which have asilane coupling agent attached to or included in the polymer backbone.These especially preferred acrylate copolymers are commerciallyavailable and can be prepared by copolymerizing the acrylate copolymermonomers with a silane coupling agent having ethylenic unsaturationwhich can undergo free radical polymerization. The amount of silanecoupling agent in the acrylate copolymer can represent from about 0.2 toabout 2.0 percent by weight. Especially preferred silane coupling agentcontaining acrylate copolymers include methyl methacrylate/acrylatecopolymers and styrene/acrylate copolymers. The silane coupling agentcontaining acrylate copolymers provide improved internal bonding betweenthe silaceous components (glass fibers and diatomacious earth) of theseparator. Battery separators including the preferred silane couplingagent containing acrylate copolymers exhibit especially superiorresistance to corrosive acids.

When combined with the fiber and filler materials, the acrylatecopolymer binders of this invention provide a separator which is acidwettable but not water wettable. By "acid wettable" it is meant that adrop of sulfuric acid solution (1.2 specific gravity) placed on thesurface of a separator of this invention will be substantiallycompletely absorbed or otherwise not discernable as a drop within aboutfive seconds or less. By "not water wettable", it is meant that a dropof water placed on the surface of the separator will not be completelyabsorbed but will be discernable as a drop for about 30 minutes andpreferably for about 60 or longer. The binder is used in amounts toprovide between about 7 to about 20 and most preferably between about8.5 to about 17 percent by weight of the finished separator.

The polymeric binders are used as latex dispersions which have an acidpH. Typical properties of separators formed with the latex disperionsinclude a tensile of about greater than about 10 lbs/in. and a punctureof more than about 400 grams.

Essentially separators of this invention are produced by forming anaqueous dispersion of the above described fiber materials, fillermaterial and binder material, sequentially adding dispersing andflocculating agents to the dispersion, applying the flocculatedagglomerates to the wire of a web forming apparatus to provide a webproduct having a thickness of about 30 mils and dewatering the product.

The preferred preparation of separators of the invention, particularlyat a pilot plant or full production scale, involves a retention acid orsystem which utilizes a charge-reverse-charge mechanism. The procedurecan be performed on standard papermaking apparatus such as afourdrinier, an inclined fourdrinier, a cylinder machine, a rotoformeror the like. According to this procedure, the pulper is first chargedwith water and the polyolefin fibers. The polyolefin fibers are clearedby circulation through a refiner usually for about 20 minutes, then thediamtomaceous earth and the polyester and glass fibers are added. Adispersion of a heavily anionic material is then charged to the pulper.The preferred anionic material is a carboxylated acrylamide and theamount used is preferably about 0.2% by weight of the total furnish. Theanionic material functions as a dispersing agent and imposes a heavyanionic charge on the fiber and filler materials. The latex binder whichis also anionic in character is then added as the last ingredient.Accordingly, unlike usual beater addition procedures, no flocculationoccurs in the pulper. It has been found that if flocculation of theingredients occurs in the pulper, shear forces generated by agitationand pumping can cause the latex particles to be dislodged from theirbonding sites on the fiber.

The unflocculated furnish is then charged to the machine chest of apapermaking machine by way of a pump--such as a fan pump. The furnish ispreferably flocculated at the fan pump by mixing a cationic flocculatingagent--preferably a polyelectrolyte--with the furnish. The amount ofcationic flocculating agent added to the dispersion at this point can beregulated to control the size of the flocculated agglomerates depositedon the wire which permits adjustment and/or control of drainage time andretention rate for optimum results. According to this procedure,retention rates of 97-98 percent can be achieved.

The flocculated agglomerates charged to the machine chest aretransferred to the moving wire of the papermaking machine as a web fordrainage of water. After draining the water by gravity, suction may beapplied to remove further water from the web. The web may then bepressed between felt rolls and dried on drying cans. Suitable dryingtemperatures are between about 230° to about 260° F. and during dryingat least some crosslinking of the binder material occurs.

Another procedure for avoiding the effect of shear forces on thematerials dispersed in the pulper involves a variation of theabove-described preferred preparation. According to this alternativeprocedure, a cationic flocculating agent is first added to the materialdispersed in the pulper to obtain retention of the binder and filler.Reduction in particle size of the flocculated material by the shearforces is controlled by adding a highly anionic colloid such asdeacetylated Karaya Gum or a high molecular weight anionic colloid afteraddition of the cationic flocculating agent. The anionic colloidprovides a negative charge on the flocculated agglomerates whichdisperses the agglomerates into smaller particles which are resistant tofurther reduction in size in response to the encountered shearingforces. The cationic flocculating agent is reintroduced to thedispersion preferably at the fan pump and the flocculated material ischarged to the machine chest, transferred to the moving cure and driedas described before. This alternative procedure is more analogous totypical beater addition procedures known to the art.

The invention as well as manners of making and using it will be morefully appreciated from the following illustrative, non-limitingExamples.

EXAMPLE 1

The following materials were added in the sequence listed to a Williams10"×12" hand sheet mold:

    ______________________________________                                        Polyolefin Fiber.sup.1.                                                                           6.96 grams (42% solids)                                   Filler 2.(Diatomaceous Earth)                                                                     14.4 grams (diluted                                                           to 3% consistency in                                                          water and defibered in a                                                      Waring Blender for 2                                                          minutes)                                                  Glass Fiber         0.81 grams (1/4" × 5.6                                                  microns)                                                                      0.81 grams (1/8" × 5.6                                                  microns)                                                  Polyester Fiber.sup.3.                                                                            0.81 grams                                                                    (defibered in                                                                 Waring Blender for 20                                                         seconds)                                                  Anionic Colloid.sup.4.                                                                            60 grams (at 0.1%                                                             solids)                                                   Polymeric Binder.sup.5.                                                                           4.05 mls (1.971 grams                                                         solid)                                                    Cationic Flocculating                                                                             0.3 mls.                                                  Agent.sup.6.                                                                  ______________________________________                                         .sup.1. The polyolefin fibers used in this and in Examples 2-4 are            synthetic wood pulp fibers having an average fiber diameter of 4.9 micron     and are supplied by Hercules Powder Company under the tradename PULPEX EA      2. The filler used in this and all other Examples is a diatomaceous eart     having a median particle size of 16 microns. The filler is supplied by        Johns Mansville under the trade name CELITE 110.                              .sup.3. The polyester fiber used in this and in Examples 2-4 is a 1/4",       1.5 denier polyethylene terephthalate fiber supplied by E. I. DuPont De       Nemours under the tradename DACRON.                                           .sup.4. The anionic colloid used in this Example was prepared by mixing       together 1.16 lbs of a carboxylated acrylamide sold under the tradename       BETZ DP791 by Betz Paper Chemical Incorporated with 44.5 gallons of water     The carboxylated acrylamide anionic colloid was added by addition method      and the dispersion was mixed with slow agitation until the colloid was        dissolved.                                                                    .sup.5. The polymeric binder used in this Example is a styrene/butyl          acrylate copolymer having a silane coupling agent attached to the             polymeric backbone and was supplied by National Starch and Chemical           Corporation under the designation 78-6170. The binder is supplied and use     as a latex dispersion and has a pH between 4.0 to 5.0, a wet weight of        8.64 lbs. per gallon, a dry weight of 4.181 lbs. per gallon, a percent        solids of 48.0%, a specific gravity of 1.036 and a Brookfield viscosity o     310 cps.                                                                      .sup.6. The cationic flocculating agent used in this Example is a             polyethyleneimine which is supplied by BASF Wyandotte under the tradename     POLYMIN P. The flocculent was prepared by diluting 5 gallons of POLYMIN P     in 45 gallons of water.                                                  

The above materials were diluted to a 1.5% consistency and drainedthrough a 72×76 plastic wire screen. Drainage time was three (3)seconds. The sheet was wet pressed and dried.

The dried separator sheet material had the following composition:

    ______________________________________                                                     Percent by Weight                                                             of Separator Material                                            ______________________________________                                        Polyolefin Fiber                                                                             13.4                                                           Polyester Fiber                                                                              3.7                                                            Glass Fiber    7.5                                                            Filler         66.3                                                           Binder         9.1                                                            ______________________________________                                    

The ohmic resistance of separators of the above composition having asheet gauge of 0.0285" was measured and the following results wereobtained:

    ______________________________________                                        20 minute ohmic resistance                                                                       0.0120 ohms                                                24 hour ohmic resistance                                                                         0.0105 ohms                                                ______________________________________                                    

Porosity characteristics of the sheet material were determined and thefollowing results were obtained:

    ______________________________________                                        Total Porosity   68.8466%                                                     Maximum Porosity 69.0021%                                                     Porosity due to Pores                                                                          0%                                                           Less Than 0.06223                                                             Micron Diameter                                                               Average Pore Size                                                                              7.767 microns                                                % of Pores Greater                                                                              7.5916%                                                     than 20 Microns                                                               ______________________________________                                    

Acid Wet Out for the sheet material was instantaneous and the sheetmaterial was not Water Wettable.

Battery cell tests with cells made with the separator material showexcellent charge acceptance, cold crank performance and no degradationunder an overcharge paste test.

The separator material was subject to refluxing sulfuric acid dichromatesolution for three hours and showed an average weight loss of from 1% to2% maximum with no blistering or delamination occurring.

EXAMPLE 2

A furnish was prepared by adding the following materials to a pulper inthe order listed. (The materials are the same as used in Example 1):

    ______________________________________                                        Water             2500   gallons                                              Polyolefin Fiber  260    lbs (dry)                                            Filler            1100   lbs                                                  Glass Fiber       280    lbs.                                                 (1/4" × 13.0 microns)                                                   Polyester Fiber   100    lbs.                                                 Annionic Colloid  172    gals. 0.3% solids                                    Binder            62     gallons                                              Cationic Flocculating                                                         Agent.sup.7.                                                                  ______________________________________                                         .sup.7. The cationic flocculating agent used in this Example and in all       following Examples is a polyamine which is supplied by Betz Corporation       under the tradename BETZ 1185. The flocculant was prepared by dilating on     gallon of BETZ 1185 in 50 gallons of water.                              

The fiber and filler materials were dispersed in the sequence notedabove to provide a uniform dispersion. The fiber and filler materialswere negatively charged by the addition of the anionic colloiddispersing agent. Addition of the anionically charged binder completedthe pulper cycle and the dispersion was then pumped to the machine chestof a rotoformer papermaking machine. The pump used to charge the machinechest was a fan pump and at the fan pump, the cationic flocculatingagent was added to and mixed with the dispersion at a rate of about 48gallons per hour. Water was added to maintain the dispersion in themachine chest at a consistency of about 1.2%.

The dispersion was fed onto the moving screen of the rotoformer toprovide a web, water was allowed to drain and then suction was appliedto the web. The wet web was then passed between a roll over felt pressand then dried by passing it over heated rolls to heat the web to about255° F.

The dried separator sheet material had the following composition:

    ______________________________________                                                     Percent by Weight                                                             of Separator Material                                            ______________________________________                                        Polyolefin Fiber                                                                             12.87                                                          Polyester Fiber                                                                               6.93                                                          Glass Fiber    13.86                                                          Filler         54.45                                                          Binder         12.7                                                           ______________________________________                                    

A battery separator prepared as described had the following properties:

    ______________________________________                                        Property            Value                                                     ______________________________________                                        Backweb t.sub.b ins 0.027                                                     Overall t.sub.b ins 0.045                                                     Weight/area g/m.sup.2                                                                             238                                                       Ohmic Resistance    0.016                                                     20 mins in.sup.2                                                              24 hours in.sup.2   0.014                                                     48 hours in.sup.2   0.013                                                     Apparent density.sup.1.                                                                           0.3952                                                    % Porosity          74                                                        Pore diameter avg.  14                                                        in microns                                                                    % Volume greater    20                                                        than 20 microns                                                               Gradient Columar density %                                                                        1.74                                                      Tensile strength lbs.                                                                             16.2                                                      % Elongation        1.0                                                       Acid Wettability    Instantaneous                                             Water Wettability   60+ minutes                                               COD                 1400 ppm                                                  Puncture Resistance Greater than 600 grams                                    ______________________________________                                         .sup.1. Measured on an Aminco Mercury Porosimeter under liquid mercury        pressure.                                                                

The battery separator was used in a lead-acid storage battery and testedfor performance according to the Battery Counsel Industry (BCI)recommended specifications for vehicular, ignition lighting and startingtypes. The results are as follows:

    ______________________________________                                        Discharge Performance Characteristics                                         ______________________________________                                        Reserve Capacity     64.6 minutes                                             25 amps at 80° F.                                                      minutes to 10.5 volts                                                         Cold Crank           8.16 volts                                               450 amps at 0° F.                                                      voltage at 30 sec.                                                            ______________________________________                                    

EXAMPLE 3

The following materials were added in the sequence listed to a Williams10"×12" hand sheet mold:

    ______________________________________                                        Polyolefin Fiber  6.96 grams (42% solids)                                     Filler            14.4 grams (diluted to                                                        3% consistency in water                                                       and defibered in a                                                            Waring Blender for 2                                                          minutes)                                                    Glass Fiber       0.81 grams (1/4" × 5.6                                                  microns)                                                                      0.81 grams (1/8" × 5.6                                                  microns)                                                    Polyester Fiber   0.81 grams (defibered in                                                      Waring Blender for 20                                                         seconds)                                                    Cationic Flocculating                                                                           0.3 ml.                                                     Agent                                                                         Polymeric Binder.sup.8.                                                                         4.05 mls (1.971 gms                                                           solid)                                                      Anionic Colloid.sup.9.                                                                          10 grams (0.1 gms solid)                                    ______________________________________                                        .sup.8. The polymeric binder used in this Example was a styrene/butyl         acrylate copolymer supplied by National Starch and                            Chemical Corporation under the designation                                    78-6132. The binder is supplied and used as a latex dis-                      persion and has a pH of 4.9, a wet weight of 8.64 lbs. per gallon,            a dry weight of 4,181 lbs. per gallon, a percent solids of 48.4,              a specific gravity of 1.036 and a Brookfield viscosity of 310 cps.            .sup.9. The anionic colloid used in this Example was prepared                 by mixing together the following materials:                                   Water 60° C.                                                                             331/3 gallons                                               Triethanolamine   17.5 cc.                                                    Ammonia           300 cc.                                                     Karaya Gum Powder 4.5 lbs.                                                    Cold water was then added to increase the volume to 50 gallons.               The weight of Karaya Gum Powder in the dispersion                         

The above materials were diluted to a 1.5% consistency and drainedthrough a 72×76 plastic wire screen. Drainage time was three (3)seconds. The sheet was wet pressed and dried.

The dried separator sheet material had the following composition:

    ______________________________________                                                     Percent by Weight                                                             of Separator Material                                            ______________________________________                                        Polyolefin Fiber                                                                             13.4                                                           Polyester Fiber                                                                              3.7                                                            Glass Fiber    7.5                                                            Filler         66.3                                                           Binder         9.1                                                            ______________________________________                                    

The ohmic resistance of separators of the above composition having asheet gauge of 0.0285" was measured and the following results wereobtained:

    ______________________________________                                        20 minute ohmic resistance                                                                       0.0120 ohms                                                24 hour ohmic resistance                                                                          0.0105 ohms.                                              ______________________________________                                    

Porosity characteristics of the sheet material were determined and thefollowing results were obtained:

    ______________________________________                                        Total Porosity   68.8466%                                                     Maximum Porosity 69.0021%                                                     Porosity due to Pores                                                                          0%                                                           Less Than 0.06223                                                             Micron Diameter                                                               Average Pore Size                                                                              7.767 microns                                                % of Pores Greater                                                                              7.5916%                                                     than 20 Microns                                                               ______________________________________                                    

Acid Wet Out for the sheet material was instantaneous and the sheetmaterial was not Water Wettable.

Battery cell tests with cells made with the separator material showexcellent charge acceptance, cold crank performance and no degradationunder an overcharge paste test.

The separator material was subject to refluxing sulfuric acid dichromatesolution for three hours and showed an average weight loss of from 1% to2% maximum.

EXAMPLE 4

The same amounts of the same fiber materials, filler, cationicflocculant and anionic colloid as in Example 3 were used to prepareSeparators A, B, C, D, E and F of this Example. However, the bindermaterial used in preparing the Separators of this Example was differentand the amount used was sufficient to provide 9, 10, 11, 12, 13 and 14percent by weight of binder in Separators A, B, C, D, E and Frespectively. The different binder material used in this Example is amethyl methacrylate/butyl acrylate copolymer supplied by National Starchand Chemical Corporation under the designation 78-6139. The binder issupplied and used as a latex dispersion and has a pH of 4.2, a wetweight of 8.94 lbs. per gallon, a dry weight of 4.326 lbs. per gallon, apercent solids of 48.4, a specific gravity of 1.070 and a Brookfieldviscosity of 310 cps. The ohmic resistance of the Separators are listedin tabular form below:

                  TABLE 1                                                         ______________________________________                                                               20 Minutes                                                                              24 Hour                                                % by Weight  Ohmic     Ohmic                                        Separator Binder       Resistance                                                                              Resistance                                   ______________________________________                                        A          9           0.0100    0.0090                                       B         10           0.0104    0.0100                                       C         11           0.0104    0.0090                                       D         12           0.0125    0.0100                                       E         13           0.0114    0.0110                                       F         14           0.0120    0.0114                                       ______________________________________                                    

Acid Wet-Out for each Separator material of Table 1 was instantaneousand each Separator was not Water Wettable.

Battery cell tests with each Separator of Table 1 show excellent chargeacceptance, cold crank performance and no degradation under anovercharge plate test.

Average weight losses for each Separator of Table 1 after three hours ofimmersion in refluxing sulfuric acid dicromatic solution were 1% to 2%maximum.

EXAMPLE 5

A furnish was prepared by adding the following materials to a pulper inthe order listed.

    ______________________________________                                        Water                                                                         Polyolefin Fiber.sup.11                                                                     240 lbs. (dry)   (620 lbs. wet)                                 Filler        950 lbs.                                                        Glass Fiber                                                                   1/4" × 12 microns                                                                     155 lbs. (dry)   (182 lbs. wet)                                 1/2" × 12 microns                                                                     155 lbs. (dry)   (193.75 lbs. wet)                              Polyester Fiber.sup.12                                                                      200 lbs. (dry)                                                                 6 lbs.  (dry)   (240 gals.)                                    Annionic Colloid.sup.13                                                                     300 lbs. (dry)   (77.5 gals.)                                   Binder.sup.14                                                                 Cationic Flocculating                                                         Agent                                                                         ______________________________________                                         .sup.11 The polyolefin fibers used in this Example are synthetic wood         fibers having an average fiber diameter of about 5-10 microns and are         supplied by Mitsui Corporation.                                               .sup.12 The polyester fiber used in this Example is a 1/4" × 1.5        denier supplied by Hoechst under the tradename TREVIRA 101.                   .sup.13 BETZ 1185 (See 4. Example 1.)                                         .sup.14 The polymeric binder used in this Example is a styrene acrylate       copolymer having a silane coupling agent attached to the polymeric            backbone and was supplied by National Starch and Chemical Corporation         under the designation 78-6187. The binder is supplied and used as a latex     dispersion and has a pH of about 4.7 a wet weight of 8.6 lbs. per gallon,     a dry weight of 3.87 lbs. per gallon, a percent solids of 45%, a specific     gravity of 1.029 and a Brookfield viscosity of 300 cps.                  

The fiber and filler materials were dispersed in the sequence notedabove to provide a uniform dispersion. The fiber and filler materialswere negatively charged by the addition of the anionic colloiddispersing agent. Addition of the anionically charged binder completedthe pulper cycle and the dispersion was then pumped to the machine chestof a fourdrinier papermaking machine. The pump used to charge themachine chest was a fan pump and at the fan pump, the cationicflocculating agent was added to and mixed with the dispersion at a rateof about 228 gallons per hour. Water was added to maintain thedispersion in the machine chest at a consistency of about 3%.

The dispersion was fed onto the moving screen of the rotoformer toprovide a web, water was allowed to drain and then suction was appliedto the web. The wet web was then passed between a roll over felt pressand then dried by passing it over heated rolls to heat the web to about255° F.

The dried separator sheet material had the following composition:

    ______________________________________                                                     Percent by Weight                                                             of Separator Material                                            ______________________________________                                        Polyolefin Fiber                                                                             12                                                             Polyester Fiber                                                                              10                                                             Glass Fiber    15.5                                                           Filler         47.5                                                           Binder         15.                                                            ______________________________________                                    

A battery separator prepared as described had the following properties:

    ______________________________________                                        Property               Value                                                  ______________________________________                                        Backweb t.sub.b ins    0.0313                                                 Overall t.sub.b ins    0.060                                                  Weight/area g/m.sup.2  196.5                                                  Ohmic Resistance                                                              20 mins in.sup.2       0.0121                                                 24 hours in.sup.2.     0.0108                                                 Apparent density       0.313                                                  % Porosity             83.61                                                  Pore diameter avg.     24.8                                                   in microns                                                                    % Volume greater       57.                                                    than 20 microns                                                               Tensile strength lbs.  11.91                                                  % Elongation           1.32                                                   Acid Wettability (seconds)                                                                           0.8 seconds                                            Water Wettability (minutes)                                                                          30 minute +                                            COD                    1972                                                   Puncture Resistance (gms)                                                                            469 gm                                                 ______________________________________                                    

The battery separator was used in a lead-acid storage battery and testedfor performance according to the Battery Counsel Industry (BCI)recommended specifications for vehicular, ignition lighting and startingtypes. The results are as follows:

    ______________________________________                                        Discharge Performance Characteristics                                         ______________________________________                                        Reserve Capacity     64.6 minutes                                             25 amps at 80° F.                                                      minutes to 10.5 volts                                                         Cold Crank           8.16 volts                                               450 amps at 0° F.                                                      voltage at 30 sec.                                                            ______________________________________                                    

The battery separators of Examples 1, 2 and 5 include the especiallypreferred acrylate copolymer having a silane coupling agent attached toor included in the polymer backbone. Separators containing theespecially preferred acrylate copolymers exhibit superior resistance tocorrosive acids and this is an important performance characteristic formaintenance free battery separators. As those in the art know, a batteryundergoes plate formation via an electrical charge during construction.Frequently during such plate formation, charging conditions may beinadvertently exceeded leading to higher than desired temperatures inthe battery. Currently available maintenance free battery separatorstend to blister or delaminate under these temperature conditions,resulting in short circuiting of the plates. A test to determine thepropensity of a separator to delaminate or blister under the abovedescribed conditions involves refluxing a sample of the separator in asulfuric acid dichromate solution for three hours. After refluxing, thesample is examined to determine if blistering or delamination hasoccurred and the degree of blistering or delamination which hasoccurred. Separators of Examples 1, 2 and 5 show no blistering ordelamination under these test conditions.

From the above description, it will be apparent that this inventionpresents to the art novel, improved, acid wettable, non-cellullosicbattery separators particularly adaptable for use in maintenance freebatteries. Moreover, the separators present the distinctive combinationof excellent electrical performance characteristics, low material andproduction costs and good physical properties and manufacturingfeatures. The separators have low ohmic resistance, low chemical oxygendemand and excellent porosity characteristics in terms of pore volumeand low average pore size generated by using a large amount of arelatively inexpensive filler. Accordingly, the novel separators of thisinvention provide unexpected advantages in terms of overall performancecharacteristics, cost and manufacture as compared to those known to theart at the time the invention was made.

I claim:
 1. A battery separator which is acid wettable but not waterwettable and which comprises a substantially uniform mixture of fromabout 5 to about 20 percent by weight of the separator of polyolefinfiber, from about 2 to about 15 percent by weight of polyester fiber,from 0 to about 20 percent by weight of glass fiber, from about 40 toabout 75 percent by weight of diatomaceous earth and from about 7 toabout 20 percent by weight of an acrylate copolymer binder whichincludes a silane coupling agent attached to the polymeric backbone,said separator being characterized by a total pore volume of about 70percent or higher with an average pore size between about 5 to about 30microns and an ohmic resistance of about 0.015 ohms per inch² or less.2. A battery separator of claim 1 which comprises from about 8 to about16 percent by weight of polyolefin fiber, from about 5 to about 12percent by weight of polyester fiber, from about 12 to about 18 percentby weight of glass fiber, from about 45 to about 70 percent by weight ofdiatomaceous earth and from about 8.5 to about 17 percent by weight ofbinder.
 3. A battery separator of claim 1 where the total pore volume isgreater than about 80 percent and the average pore size is between about15 to about 30 microns.
 4. A battery separator of claim 1 where thebinder is a copolymer of methyl acrylate or ethyl acrylate or propylacrylate or butyl acrylate or mixtures of these.
 5. A battery separatorof claim 1 where the binder is a copolymer of methyl methacrylate andethyl acrylate, methyl methacrylate and butyl acrylate, styrene andethyl acrylate, styrene and butyl acrylate and mixtures of these.
 6. Abattery separator of claim 1 where the diatomaceous earth is a fluxcalcined grade diatomaceous earth.
 7. A battery separator of claim 1where the total pore volume is between about 70 to about 90 percent. 8.A battery separator of claim 1 where the glass fiber includes glassfibers having a length of about 1/2 inch and a diameter between about 2to about 15 microns.
 9. A battery separator of claim 1 where the glassfiber includes glass fibers having a length of about 1/2 inch and adiameter between about 2 to about 15 microns in combination with glassfibers having a different length and/or a different diameter.
 10. In abattery with an electrolyte and a positive plate and a negative plateseparated by a battery separator, the improvement where said separatoris a separator of claim 1.